This invention relates to a color image pickup device, and more particularly to a color image pickup device of the single plate type, which uses a solid state image sensing element with a complementary color separation mosaic filter.
To construct a single plate type color imaging device using one solid state image sensing element, it is necessary to apply a mosaic filter for color separation to the imaging surface. This is needed for separating the output signal from solid state image sensing element into a plurality of color components. The mosaic filter has a mosaic array of picture elements (pixels) for color components. The mosaic filter is categorized into a primary color type filter and a complementary color type filter. The primary color type filter is constructed based on the combination of three primary colors, red (R), green (G), and blue (B). The complementary color type filter is based on the combination of a complementary color such as yellow (Ye; allows R and G to transmit therethrough) and cyan (Cy; allows B and G to transmit therethrough) and others, and white (W; allows R, G and B to transmit therethrough, viz. it is transparent) or G. The former is inferior to the latter in sensitivity and resolution. This is because the latter has a high light transmittivity.
The color image pickup device of the single plate type using the complementary color filter is described by Sone et al. in "Color Image Pickup System Using the Single Plate Type CCD in Field Storage Mode", Journal of the Institute of Television Engineers of Japan, Vol. 37, No. 10 (1983).
FIG. 1 shows a block diagram of this conventional single-plate type color image pickup device. The field storage mode means a read-out mode in which the pixel information in two horizontal lines of the filter are read out as the pixel information of one horizontal scanning line, and the two lines are present vertically shifted by one line between A (odd) and B (even) fields. The field storage mode is superior to the frame storage mode in that the former has fewer afterimages. The pixels of the color separation filter are arrayed so that the luminance signals are equal in the each field and line, and at least two kinds of color component information can be obtained from two horizontal lines. This is made so that if two adjacent pixels arrayed in the vertical direction are read out as one pixel, no problem arises. To secure the frequency band of the luminance signal, the pixel array in the horizontal line is repeated every other columns. To this end, a comcolor separation mosaic filter is employed for the color separation filter.
The mosaic filter is made up of basic patterns of pixels which are repetitively arranged vertically and horizontally. In the basic pattern, pixels of Cy, Ye, magenta (Mg; allows R and B to transmit therethrough), and G are arrayed in 4 lines and two columns, as shown in FIG. 2.
An optical image of an object to be imaged, which comes from imaging lens 6, is incident on the imaging surface of a solid state image sensing element, such as CCD 10, through color mosaic filter 8. The imaging signal output from CCD 10 is applied to low-pass filters (LPFs) 12 and 14, and band-pass filter (BPF) 16. The passing bands of LPFs 12 and 14 are 3 MHz and 0.5 MHz, respectively. The center frequency of BPF is 3.58 MHz, and its band width is approximately 1 MHz.
The color elements of color separation filter 8 are arrayed as shown in FIG. 2. Therefore, for each horizontal scanning line, the luminance signal is produced, which contains color components as given by ##EQU1##
LPFs 12 and 14 produce broad-band (i.e. wide-band) luminance signal Y.sub.H and narrow-band luminance signal Y.sub.L. Broad-band luminance signal Y.sub.H is input through gamma (.gamma.) compensation circuit 18 to video processor 20 for processing the composite video signal. The output signal from BPF 16 is input to adder/subtractor circuit 26 through demodulator 22 and LPF 24. In demodulator 22, an odd-numbered column signal is subtracted from an even numbered column signal, and alternately produces color difference signals as given below. The line designated as an n line in FIG. 2 provides a color difference signal as given by ##EQU2## The n+1 line provides a color difference signal as given by ##EQU3##
The narrow-band luminance signal Y.sub.L output from LPF 14 is also input to adder/subtractor 26. The color difference signals necessary for the composite video signal are R-Y and B-Y signals. Adder/subtractor circuit 26 multiplies the color difference signals 2B-G and 2R-G, and narrow-band luminance signal Y.sub.L by appropriate coefficients, and adds/subtracts these signals to produce color difference signals R-Y and B-Y. Demodulator 22 alternately produces the color difference signals 2B-G and 2R-G signals every scanning line. Accordingly, adder/subtractor 26 also alternately produces the color difference signals B-Y and R-Y every scanning line. For this reason, the output signals of the two lines from adder/subtractor circuit 26 are averaged by using 1H (one horizontal scanning period) delay circuit 28 and line select circuit 30. The color difference signal of each line is delayed by the 1H period, and together with the color difference signal of the next line, is output from line select circuit 30.
The color difference signals R-Y and B-Y output from line select circuit 30 are modulated by modulator 32 (with a center frequency of 3.58 MHz), to form a color subcarrier signal.
The color subcarrier signal is supplied to video processor 20. The video processor 20 forms a composite video signal using the color subcarrier signal, the broad-band luminance signal Y.sub.H ' output from gamma compensation circuit 18, and the synchronizing signal.
In the conventional color image pickup device, the gamma compensation is applied only for the broad-band luminance signal Y.sub.H. Since the luminance signal has a great correlation to the G signal, therefore, it can be considered that the gamma compensation was applied almost exclusively to the G component of R, G and B components. Therefore, the color image pickup device with unsatisfactory hue reproduction is unable to image sense the color components of all colors accurately.
The color filter of the complementary color type as shown in FIG. 2 involves the problem of high light green or green highlights.
The saturation characteristics of the color components Ye, Mg, Cy, and G, which constitute the complementary color filter, are different from one another, as shown in FIG. 3A. The primary color signals of the additive color system are obtained from the output signals Ye, Mg, Cy and G of the image sensing elements with the complementary color filter, by an appropriate operation. For example, for obtaining the R signal, the following equation (4) is used. EQU (Ye+Mg)-(Cy+G)=2R-G (4)
Rearranging the above equation, we have EQU Ye+Mg-Cy=2R (5).
As seen from the equation (5), in the range where any component signals of Ye, Mg, Cy and G are not saturated, viz. the intensity of incident light is less than I1, the amplitude of the signal R is proportional to the incident light intensity as shown in FIG. 3B. When only the Ye signal is saturated, the proportional coefficient of the R signal is decreased. When the Mg signal enters the saturation range, viz. the incident light intensity exceeds I2, only the component of Cy increases, so that the R signal decreases as seen from the equation (5).
When the intensity of light changes during the imaging of the same object, the primary color component, for example, the R signal, must be proportional to the incident light intensity. However, as described above, if the incident light intensity exceeds a predetermined value, the proportional relationship does not hold, so that the R signal is smaller than its proper value. Therefore, the primary colors other than the R are more intensive, presenting light sea green of the reproduced image. In other words, the so called high light green phenomenon occurs.
To prevent this phenomenon, the difference of the saturation characteristics of the complementary color component signals is compensated for using amplifiers with different amplification factors. An conventional example of such compensating approach is shown in FIG. 4. As shown, the output signal of CCD 10 is selectively applied to one of saturation type amplifiers 36a to 36d with different saturation levels through multiplexer 34. The CCD 10 is provided with a complementary type color filter using four colors Ye, Mg, Cy and G, which is of the above-mentioned type. As well known, the saturation type amplifier is saturated in its output signal when its input signal exceeds a predetermined value. The multiplexer 34 is switched every pixel, to allow the pixel signals of the color components Ye, Mg, Cy and G to be respectively supplied to saturation type amplifiers 36a to 36d. The input/output characteristics of the amplifiers 36a to 36d as for the color components Ye, Mg, Cy and G are shown in FIG. 5. As shown in FIG. 3A, of those color component signals Ye, Mg, Cy and G, the Ye signal are saturated for the least intensity of the incident light. Since the saturation type amplifiers 36a to 36d have different input/output characteristics, when the Ye signal is saturated, the remaining three color components of Mg, Cy and G are simultaneously saturated. In other words, the incident light intensities at which the respective color component signals output from amplifiers 36a to 36d are saturated, are the same. The output signals from the amplifiers 36a to 36d are applied through multiplexer 38 to LPFs 12 and 14, and BPF 16. The multiplexers 34 and 38 are interlocked with each other.
High cost and high performance multiplexers must be used for the multiplexer of this saturation difference compensating device, because a high speed operation is required for the multiplexers. Further, the amplifier is used for each color component, resulting in complexity of the circuit arrangement.
Size reduction is required for the camera head. This is true particularly for the color image pickup device applied for medical equipment such as electronic endoscopes. To meet this requirement, the camera head containing CCD, for example, and the camera controller for processing the picked up image signal are provided separately. Nevertheless, the size reduction of the camera head is unsatisfactory, because the driver IC for CCD, together with CCD, is contained in the camera head. This IC is also contained in, for example, the 8-pin DIP package, since it emits a large amount of heat. Usually, it is necessary to apply a multi-phase (not less than three-phase) driving signal to CCD, and hence at least three ICs are required. This brings about the increased size of camera head. Additionally, if the heat of the IC is transferred to the CCD, the dark current of the CCD increases, causing noise. For this reason, the drive IC must be separated from the CCD. This fact also leads to the increase of the camera head size.